The origin of the magnetic fields observed in galaxies and galaxy clusters is one of the outstanding problems in cosmology. Although the amplification of weak “seed” magnetic fields, via the turbulent dynamo mechanism in conducting fluids or plasmas [1], is quite well understood, how the seed itself forms has remained a serious challenge. In ideal fluid dynamics, a topological constraint prohibits the emergence of a magnetic field, or, more generally, a vorticity, from a zero-field state. This leaves the question: How can an initially magnetic-field-less cosmological fluid give rise to a nonzero seed field?

Various scale-specific solutions to this problem have been proposed, usually employing non-ideal fluid dynamics. In an article appearing in *Physical Review Letters*, Swadesh Mahajan of the University of Texas, US, and Zensho Yoshida of the University of Tokyo, Japan, demonstrate a universal vorticity-generating mechanism using ideal, but relativistic, fluid dynamics. They show that special relativity breaks the topological constraint against the emergence of vorticity, even for mildly relativistic fluid flows. Their finding could lead to a better understanding of the origin of magnetic fields in the Universe. – *Jerome Malenfant*

[1] L. M. Widrow, Rev. Mod. Phys. **74**, 775 (2002).